Neural Wave Functions for Superfluids

TL;DR

This paper advances the modeling of superfluid systems by modifying the Fermionic neural network wave function Ansatz with an antisymmetric geminal power approach, achieving more accurate results for the unitary Fermi gas.

Contribution

It introduces a new AGPs FermiNet wave function Ansatz that improves accuracy in superfluid systems and proves it is a strict generalization of the original FermiNet architecture.

Findings

The AGPs FermiNet outperforms original FermiNet in paired systems.

Results are more accurate than fixed-node diffusion Monte Carlo.

The new Ansatz is a strict generalization of the original FermiNet.

Abstract

Understanding superfluidity remains a major goal of condensed matter physics. Here we tackle this challenge utilizing the recently developed Fermionic neural network (FermiNet) wave function Ansatz [D. Pfau et al., Phys. Rev. Res. 2, 033429 (2020).] for variational Monte Carlo calculations. We study the unitary Fermi gas, a system with strong, short-range, two-body interactions known to possess a superfluid ground state but difficult to describe quantitatively. We demonstrate key limitations of the FermiNet Ansatz in studying the unitary Fermi gas and propose a simple modification based on the idea of an antisymmetric geminal power singlet (AGPs) wave function. The new AGPs FermiNet outperforms the original FermiNet significantly in paired systems, giving results which are more accurate than fixed-node diffusion Monte Carlo and are consistent with experiment. We prove mathematically that the new Ansatz, which only differs from the original Ansatz by the method of antisymmetrization, is a strict generalization of the original FermiNet architecture, despite the use of fewer parameters. Our approach shares several advantages with the original FermiNet: the use of a neural network removes the need for an underlying basis set; and the flexibility of the network yields extremely accurate results within a variational quantum Monte Carlo framework that provides access to unbiased estimates of arbitrary ground-state expectation values. We discuss how the method can be extended to study other superfluids.